The subject invention relates generally to the field of valves for controlling plastic extruding systems. In particular, the present invention is directed to a sliding ring valve having a design that provides a longer operating life than is achieved with similar, conventional designs.
A variety of devices have been used for mixing and fluxing thermoplastic materials. Conventionally such systems have used extruders with a helical screw rotating within a cylinder or barrel. The extruder barrel has an input port and a discharge port. A high pressure injection molding apparatus cooperates with the extruder at the output side of the discharge port.
The extrusion and molding processes are sequential. First, solid thermoplastic pellets are fed into the extruder barrel though the input port. The rotation of the helical screw, in cooperation with the inner wall of the cylinder, forces the thermoplastic pellets though the extruder. The pellets are heated and melt as a resin. In this form the material moves along the extruder barrel. The molten plastic resin is then fed through the valve. At this point, a volume of air or other material is displaced in the cylinder forcing the screw to retract back to a pre-set position. The screw rotation then stops. With the extrusion step completed, the molten material is injected into the injection molding apparatus.
Providing an even flow of molten material with the desired consistency for the injection molding apparatus is crucial to the efficiency and reliability of the extrusion process. This is critical to the quality of the resulting product. The valve is thus, a key element in any resin handling and extruding process.
The valve is mounted at the downstream or leading end of the helical screw during the injection process. The leading end of the screw is located near the discharge port of the extruder barrel. The valve is normally closed, but is forced open during the extrusion step to allow molten resin material to flow through the valve and thereby displace a volume of air or other material in the cylinder. This procedure allows for the reciprication or retraction of the molten plastic to assure homogenation and composition consistency. The valve also automatically closes following the extrusion step to prevent back-flow of plastic through the extruder during the high pressure injection molding process. The efficient operation of the valve is crucial to the injection molding process.
Non-return check valves and check valves (sometimes referred to as screw tips) are used on most injection and injection blow molding machines. There are a few exceptions such as rigid PVC and thermoset resins. However, in some applications, sliding ring valves are used with rigid PCV. Most conventional check valves used with plastic extruding systems can be divided into two categories: (a) ball check valve; and, (b) sliding ring valve. Of the two, sliding ring type valves are more commonly used. With both types of valves, the forward movement of a plasticating screw is required by the dynamics used for the shut-off operation.
There are advantages and disadvantages to both types of valve. The advantages of the ball check valve are: (a) an efficient, positive shut-off control; (b) efficient shot control; and, (c) a front discharge ball check valve is often less expensive than a sliding ring valve. However, there are also disadvatages to the use of ball check valves. These include: (a) a less streamlined geometry, causing more degradation of heat-sensitive materials; (b) more barrel wear and galling; (c) the side discharge type valves are more expensive than a sliding ring type valves; (d) greater pressure drops occur, therefore creating more heat; (e) poor performance occurs in vented operations; and, (f) ball check valves are much harder to clean.
One derivation of the aforementioned ball check design is the Spirex poppet valve. This valve is described on page 53 of the 1997 Spirex publication entitled Plasticating Component Technology incorporated herein by reference as providing background material. This valve can be designed with flutes or grooves for increased mixing capability. The valve also has excellent shut-off characteristics, and is usually used with low viscosity resins. However, this valve is relatively complex.
For this and a number of other reasons, the sliding ring valve is more widely used than the ball check valve in the plastic extrusion molding industry. Of crucial importance is the fact that sliding ring valves are less expensive than side discharge ball check valves. Other advantages of sliding ring valves include: (a) greater streamlining for less degradation of processed materials; (b) best for heat-sensitive materials; (c) less barrel wear; (d) less pressure drop across the valve; (e) well-suited for vented operation; and, (f) easier cleaning than with ball check valves. However, there are some disadvantages to the sliding ring valve, including: (a) less positive shut-off control, especially in 4xc2xd inch diameter and larger size valve; (b) less shot control than is provided with ball check valves; and, (c) sliding ring valves are more expensive than front discharge ball check valves.
There are many variations to the sliding ring design. However, there are certain similarities; as the name implies, a sliding ring is used to provide the shut-off operation. The most common valve used is the three-piece design. This valve is simple, versatile, and relatively inexpensive. Some of the most common variations for sliding ring valves are depicted on pages 53-56 of the aforementioned Spirex publication. These include: (a) a three piece, free flow valve; (b) the four-piece Mallard design; (c) the CMD design; (d) the Castle design; and,(e) the Spirex HMV design. Other designs are also available. One example is the Spirex pin valve design, described on page 55 of the Spirex application materials. Another Spirex design is the xe2x80x9cAuto-Shutxe2x80x9d valve, described in U.S. Pat. No. 5,164,207, and incorporated herein by reference. This design uses a spring and a thrust bearing to carry out a highly consistent closing operation of the subject valve.
All of the aforementioned valves have advantages and disadvantages. As valve design becomes more complex, the valve must become more expensive since a greater number of parts, and thus, a greater number of manufacturing processes are used to produce the valve. Further, all of the aforementioned valves suffer from wear due to the high pressures involved and the potentially corrosive materials being used. The more complex valves often are more vulnerable to both wear and corrosive degradation due to the higher complexity in the geometries of such valves. Thus, the simple three-piece sliding ring valve is very often used because of its longevity and/or price.
Nonetheless, even the simple, three-piece valves wear out. This wear is caused by the rotation of the screw of the injection molding machine causing the ring to rotate against the retainer. The retainer is usually the part of the valve that wears out most rapidly. Further, most retainers have flutes that provide a flow path for the resin. These flutes reduce the area of contact between the ring and the retainer. It is well-known that contact pressure rises with the reduction in the contact area, there is less metal to wear away, and that metal is under higher contact pressure. The result is a higher degree of wear and a shorter valve life.
One solution is the use of retainer made from a more wear-resistant material. However, the price of making such a retainer is extremely high. As a result, the valve price becomes much higher. Such a modification does not necessarily solve the problem of longevity since wear-resistant material is usually sensitive to torque fracture. This is a factor that cannot be avoided since the rotating assembly of the valve is subjected to high torque as part of the process of moving the screw to extrude the plastic resin.
Another conventional solution is the use of a four-piece (rather than three-piece) valve to reduce wear. The fourth piece is a wear-resistant material while the rest of the retainer is made of a more conventional material. A drawback is that this additional part increases the price of the valve due to the addition of the wear-resistant part.
Accordingly, there is still a need in the plastic extrusion industry for a simple, inexpensive valve that is highly resistant to wear so as to have a high production life. Such a valve must also be highly resistant to accumulations of resin, and the degradation caused by such accumulations. Such a valve must also be highly resistant to fracturing caused by high torques, or the burning of resins at high-pressure valve interfaces.
Accordingly, it is one object of the present invention to provide a valve that has improved performance over conventional three-piece valves.
It is another object of the present invention to provide a valve that is less expensive than conventional high performance valves.
It is an additional object of the present invention to provide a three-piece valve capable of easily processing heat-sensitive resin without burning the resin.
It is a further object of the present invention to provide a valve that does not have cold-start breakage problems.
It is still a further object of the present invention to provide a valve that is not conducive to accumulations of resin.
It is still another object of the present invention to provide a valve with extremely high flow-through characteristics.
It is yet an additional object of the present invention to provide a valve with an extended production life.
It is again a further object of the present invention to provide a valve with decreased wear at high pressure contact surfaces.
It is still another object of the present invention to avoid labor-intensive structures such as flutes in the manufacturer of valves for injection molding systems.
It is still another object of the present invention to provide a valve design that is sufficiently flexible so that the valve can be modified to a four-piece design.
It is yet an additional object of the present invention to provide a three piece valve with good resin flow control characteristics.
It is again a further object of the present invention to provide a three-piece valve, which is highly wear-resistant, and has the same high flow area as an F-flow valve.
It is still another object of the present invention to provide a three-piece valve having similar discharge characteristics for flushing front end components as are present for conventional side-discharge valves.
It is again a further object of the present invention to provide a three-piece valve that minimizes mechanical interlock between the retainer and the resin.
It is still another object of the present invention to provide a three-piece valve having the advantages of increased contact between the ring and the resin being handled by the valve.
It is yet a further object of the present invention to provide a three-piece valve having an extended production life using materials standard with conventional valves.
These and other goals and objects of the present invention are achieved by a valve for extruding molten material from a plasticating screw and a barrel arrangement. The valve is arranged at a leading edge of the screw and operates proximate the output port of the barrel. The valve has a leading portion and a trailing portion, and consists of a retainer arranged proximate the leading portion. A seat is arranged opposite the retainer and proximate the trailing portion of the valve. The seat includes a second contact surface arranged for a high pressure interface. A sliding ring is arranged to move between the retainer and the seat. The sliding ring includes a plurality of apertures arranged for passing the molten material there through, and has a third and fourth contact surface arranged for high pressure interfaces. These contact surfaces correspond to the first and second contact surface respectively.